1. Field of the Invention
This invention relates generally to a high-pressure mercury vapor discharge lamp coated with a particular phosphor blend to improve the emission color response when excited by the ultraviolet radiation generated from the mercury vapor discharge. More particularly, an improvement is provided in the combined lamp lumen output and color rendering index compared with conventional lamps utilizing either a phosphor coating of europium-activated yttrium vanadate phosphate phosphor alone or with other known color-correcting phosphor additives.
2. Description of the Prior Art
The arc discharge in a high-pressure mercury vapor lamp (HPMV) produces lines of varying energy in the ultraviolet, blue and yellow portions of the spectrum. Major radiations occur at 405, 436, 546, and 578 nanometer (nm.) in the visible spectrum, and also at 254, 296, and 365 in the ultraviolet. Because of the lack of radiation in the red (about 600 nm.) region of the color spectrum, the high-pressure mercury lamp emits a bluish light. This absence of red causes most color objects to appear distorted in color values. The red-deficiency in color rendition of HPMV lamps are much improved by covering the inside of the outer envelope with a red emitting phosphor excited by the ultraviolet energy generated by the mercury arc. The red phosphors commonly used in HPMV lamps are tin (stannous Sn.sup.2.sup.+) activated strontium orthophosphate (U.S. Pat. No. 3,110,680, Koelmans et al, June, 1957) and manganese-activated magnesium fluorogermanate (U.S. Pat. No. 2,748,303, Thorington, May, 1956). More recently, europium-(Eu.sup.3+) activated yttrium vanadate and europium-activated yttrium vanadate phosphate phosphor (T. W. Luscher and R. K. Datta, Illuminating Engineering, Vol. 65, No. 1, Jan., 1970, pgs. 49-53) have found extensive use in high-pressure mercury vapor lamps. These phosphors emit in the red portion (about 600-650 nm.) of the color spectrum, thus producing a color-corrected mercury vapor discharge lamp but at x and y chromaticity values too far removed from the black body locus line. In a recent lamp development, the color correction is provided with a blend of yttrium vanadate phosphor or yttrium vanadate phosphate phospher containing magnesium fluorogermanate or magnesium arsenate and which is deposited as a layer upon a non-luminescent underlayer of silica, titania, magnesia, or alumina. This composite coating is said to provide color rendition similar to that of an incandescent lamp at high efficiency.
Utilization of cerium-activated yttrium aluminate phosphor as the only luminescent coating for HPMV lamps is also known as described in German Pat. No. 2,250,231. In said embodiment, the phosphor emission response to 436 nm. excitation from the mercury arc discharge has maximum intensity at about 5,420 Angstrom wave length with a half width of approximately 100 Angstroms and was further characterized as having a satisfactory temperature dependence. The chemical composition of said phosphor material is reported to have a ratio of yttrium oxide to aluminum oxide in the range from 1:5/3 to 1:3 with the cerium level ranging from 0.1 to 7.5 weight percent of the total weight of yttrium and aluminum oxides. Said phosphor material was also reported to have been prepared by firing the oxide mixture or source compounds for the essential metal ions utilizing an ammonium chloride flux.